[MACE]

Shot Peening Discussion

I thought some might be interested in learning a bit about shot peening.

Shot peening is a process where the part is blasted with steel balls (shot) to form small dents in a uniform maner on the part surface.

Shot peened parts are not stronger* than non-peened parts. The advantage of shot peening is increased fatigue life**.

By battering the surface with steel shot, the top .005-.030' of the surface is compacted creating a compressive layer of material.

The compressive layer is important because a crack will only grow if it is pulled apart (put in tension). If a cracked material is only pushed together (loaded in compression) the crack will never grow.

Cracks almost always initiate on the surface of parts. The compressive surface layer will go a long way to preventing these cracks from forming and growing.

*Strength is how much load you can put on a part before it deforms excessively or breaks. Example: pull on a piece of wire until it breaks into two pieces.
**Fatigue life is how many times you can repeat a loading cycle before the part will crack and fail. Example: bend a piece of wire back and forth until it breaks at the bend.​

 

[MACE]

Connecting Rods:

Any machining after peening negates the effect of peening and might even result in worse properties at the transition of peened and unpeened. So if you are going to polish (to remove stress concentrations), do it BEFORE shot peening.

"If peening does not increase strength, why should I do it on my high performace rods?"
Any time you increase engine speed you greatly increase the inertial loading of your pistons at TDC and BDC. Those loads are the fatigue loads that work your rods. The factors of fatigue calculations are the peak tension, peak compression and number of cycles. All of these go up tremendously when you increase revs. If you only increase combustion pressure, you are not really doing a whole lot to the fatigue spectrum. It's the revs that kill the reciprocating assembly.

 

[MACE]

BTW, it is typical in aircraft structure to shot peen aluminum and titanium parts as well as steel. It is particularly beneficial for aluminum machined parts. I've always wondered why we don't hear of more aluminum automotive parts being shot peened.

 

[4agze52]

i just got my rods back from the machine shop for shot peening came out really good only cost me $40 too for all four

 

[BRANDO0o0O]

If you wanted to shotpeen and cryo-treat rods, wouldnt you shotpeen then cryo-treat? so that the cryo-treating relieves any stress the shot-peening causes?

Brando

 

[GaToy]

Yeah, we have to shop peen the flap track on the C5's (the aircraft I work on). But it's not shot on. We have little flapper wheels with the balls on the end of it that strike it over and over. The inside of the landing gear bearings are shot.

~C.C.~

 

[GaToy]

Actually, I haven't ever heard of aluminum being shot peened. The stuff we do is steel. But thats not to say it isn't done.

~C.C.~

 

[ITA-MR2]

Actually, I haven't ever heard of aluminum being shot peened. The stuff we do is steel. But thats not to say it isn't done.

~C.C.~

You can shot peen any metal, with mixed effects. It is kind of like a micro-forging. Hammering on metal (to a limited extent) actually makes it less prone to cracking by aligning the molecules. Ever hear of the old sword making technique of folding the metal dozens of times?? Same principle, just on a much smaller scale.

There are lots of paths to making metal stronger; cryo, heat treating / annealing, folding, forging, shot peening, nitriding, etc. They all do the same thing, albeit by different means.

One good way to illustrate this is with soft drawn copper. Take a fresh piece of copper wire and bend it back and forth several times. Each time you bend it, it becomes a little bit stiffer. Then, after you pass the maximum strength, it will break.

 

[blind monkey]

shot peening aluminum and steel on aircraft is usually done with what GaToy describes as a flapper wheel, its called flap peening and its done to stress relieve certain materials after working with them(drilling, cutting, blending, etc) as certain alloyed materials are more prone to cracking from working them.

 

[MACE]

We use flap peening on repaired or spot faced areas that have removed the peen affect from the initial processing of the part.

Here at the "big house" we shot peen everything. Don't leave your sandwich laying about or it might get blasted <grin>.

I don't know much about cryo. It sure sounds exotic though...

 

[BRANDO0o0O]

I'll be cryo-treating most of my new engine.. havent decided if I'm going to do everything or just the short block..

Brando